Differential gear assembly

ABSTRACT

A limited slip differential gear assembly has a rotating housing containing meshing spur gears each turning a gear element which is externally helically toothed. Each gear element meshes with its own externally-toothed gear member which has a larger diameter and drives one of two output shafts between which differential movement is to take place. The meshing helical gear teeth have the characteristic that the efficiency of drive transmission in opposite directions is different and this provides the limited slip. As the axes of rotation of all rotating parts of the assembly are parallel, reduced constructional and operating costs result.

FIELD OF THE INVENTION

This invention relates to a differential gear assembly as is used totransmit drive torque from a power source to driven road wheels of avehicle, and is more specifically concerned with a differential gearassembly having resistance to wheel spin and generally referred to inthe art as a limited slip differential.

STATE OF THE ART

The conventional differential gear is a `free` differential whichdivides the driving torque equally between the driven ground wheelswhilst compensating for differences in the ground speed of the wheelscaused by the vehicle cornering or surface irregularities of the ground,or variations in tire sizes.

The free differential is a standard assembly on all but the moreexpensive motor vehicles. However it has the disadvantage that a loss oftraction and driving torque on one wheel results in a corresponding lossin driving torque on the other driving wheel. Surplus torque from thepower source will only generate spin in the wheel which has losttraction.

There have been many proposals to design a assembly which would permitunbalanced output torques in the event of loss of traction at one wheel.One such assembly is described in Gleasman U.S. Pat. No. 2,859,641 andis commercially known as the TORSEN differential. This differentialutilises inefficient gearing combined with internal friction to generatetorque imbalance between output shafts when one driving wheel cannotmaintain a required driving torque due to poor ground adhesion.

The limited slip differential assembly taught by Gleasman in the abovepatent is a complex structure reliant upon efficiency of drivecharacteristics of worm-wheel gearing or what may be better described asmodified crossed helical gearing. The number of tooth engagements issuch a layout is limited and this combined with the nature of contactbetween the teeth of crossed-helical gearing significantly limits theload capacity of such differentials. In recent times the TORSENdifferential assembly has found application in vehicles having constantfour-wheel drive where axle loading is obviously much less than inconventional two-wheel drive vehicles.

In our earlier U.S. Pat. No. 5,071,395 hereby inserted by way ofreference, is described a non-slip differential gear assembly in whichintermediate shafts mounted in a housing each carry gearing in mesh withrespective axle gears. The set of meshing gears to one output shaft areconventional spur or helical gears having teeth of opposite handrespectively, and the set of meshing gears to the other output shaft arehelical gears of the same hand having zero or a very low efficiency ofdrive between them. In the assembly described in our above patent,reliance is placed on the zero or low efficiency of drive between theset of helical gears of the same hand to prevent wheel spin, whilst thedifferent rotational characteristics of the two sets of meshing gearsallows differentialling between axles to occur. The inherent asymmetryof this differential assembly of the prior art, has not found commercialacceptance in private motor cars or trucks as difficulties in itsperformance reliability and handling characteristics have posed problemswhich have not yet been solved.

OBJECT OF THE INVENTION

An object of this invention is to provide an improved limited slipdifferential gear assembly.

SUMMARY OF THE INVENTION

In accordance with the present invention a limited slip differentialgear assembly comprises a housing mounted for rotation about a firstaxis, means for coupling a rotational drive to said housing, two coaxialand spaced gear members having external teeth and respectively mountedin opposite sides of said housing, and pairs of gear elements meshingrespectively with each of the gear members and interconnected by spurgears which rotate with respective gear elements; in which gear assemblythe axes of rotation of the gear members, the gear elements and spurgears extend parallel to one another and to the axis of rotation of thehousing, and the gear elements and gear members have meshing helicalteeth of the same hand with the gear members being of larger diameterthan the gear elements.

The advantage of having the diameter of the gear member larger than thatof the gear element in mesh with it, drive is transmitted moreefficiently from the gear member to the gear element than from the gearelement to the gear member. It will be noted that the assembly of theinvention relies upon the use of parallel shafted, helical gearing ofthe same hand. However this is not done because of its rotationalcharacteristics but because of the efficiency of drive characteristicsbetween a gear member and a meshing gear element when they exhibit adrive ratio other than 1:1. Such characteristics were previously thoughtonly to be available in worm-wheel and crossed helical gear drives.

A critical dimension of a differential gear assembly if it is to befitted into a domestic motor vehicle, is the diameter of its housing asthis has to meet the ground clearance requirements of the vehicle. Theuse of parallel shafted gearing in the assembly of the invention allowsthe development of many more tooth contacts in a given diameter of gearhousing of comparable diameter enclosing a crossed helical gearing. Thisenables the development of a limited slip differential gear assemblyhaving similar characteristics to the TORSEN differential assembly buthaving greater strength making is adaptable to a wider range ofvehicles.

INTRODUCTION TO THE DRAWINGS

The invention will now be described in more detail, by way of examples,with reference to the accompanying largely diagrammatic drawings, inwhich:

In the drawings

FIG. 1 is a vertical longitudinal section through a first embodiment ofdifferential gear assembly using two helical gear elementsinter-connected by spur gearing;

FIG. 2 is a vertical section through FIG. 1 and taken along the line andin the direction indicated by the arrows II--II in FIG. 1;

FIG. 3 is a schematic vertical section through a second embodiment ofdifferential gear assembly using eight helical gear elements arranged infour pairs, each pair being inter-connected by spur gearing;

FIG. 4 is a vertical cross-section showing how the eight gear elementsof FIG. 3 are arranged in two groups of four around two coaxial, spaced,helical gear members;

FIG. 5 is a detail drawing of part of FIG. 3 and shows how two of thegear elements associated with respective gear members, areinter-connected by spur gearing;

FIG. 6 is a second detail drawing corresponding to FIG. 5 but showinggear elements and gear members modified to generate self-cancellingaxial thrusts;

FIGS. 7 and 8 are cross-sections through a housing of the gear assemblyof FIG. 3 after removal of the gear members and gear elements, thecross-sections being respectively taken along the lines and in thedirections indicated by the arrows VII--VII and VIII--VIII in FIG. 3;and,

FIG. 9 is a set of curves of mechanical efficiency (η) plotted againsthelical lead angle (β) for different pairs of meshing helical gears eachpair having a different ratio of the gear diameters, the curves showinghow the resistance to drive transmission in one direction through thepair of gears is different from the resistance to drive transmission inthe opposite direction.

DESCRIPTION OF FIRST EMBODIMENT

The layout illustrated in FIG. 1 of the present application, uses asingle pair of planetary gear elements and has a generally symmetricallayout.

The assembly comprises a housing 1 provided by an annular casing 2 towhich are bolted at 3 respective end plates 4 and 5 containing journals6 and 7. A web plate 8 is clamped at its edge between the end plate 4and the casing 2 and provides three apertures 9,10 and 11 serving asjournals. The aperture 10 is axially aligned with the journals 6 and 7on a first axis, and the other two apertures 9 and 11 respectivelyprovide journals which define a pair of second parallel axes displacedfrom and parallel to the first axis.

A support web 12 integrally formed with the end plate 2 extends acrossthe assembly parallel to, and spaced from the web plate 8. The supportweb 12 is formed with three apertures 13,14 and 15 providing respectivejournals and which are respectively aligned with the apertures 9,10 and11 of the plate 8.

A pair of meshing spur wheels 16 and 17 are located in the space betweenthe plate 8 and the web 12 and are keyed to respective shafts 18 and 19which are supported on the second axis by the journals provided by thepairs of apertures 9 and 12, and 11 and 15, respectively.

The shaft 18 carries in the space between the plate 8 and the end plate4 a helically-toothed worm gear element 21. The end of the the shaft 18is held in a journal provided by an aperture 22 in the end-plate 4. Insimilar manner, the shaft 19 carries a helically-toothed worm gearelement 23 in the gap between the web 12 and the end plate 2, and isjournalled into an aperture 24 provided in the end plate 2.

Two half-shafts 26,27 between which differential movement is to occur,respectively extend coaxially from opposite sides of the housing 1. Theshaft 26 carries a helically-toothed gear member 28 which meshes withthe gear element 21 to form a first externally and helically-toothedgear unit, and the shaft 27 carries a helically-toothed gear member 30which meshes with the gear element 26 to form a second gear unit. Theshaft 27 is carried in the journals provided in the apertures 7 and 14,and the shaft 26 is carried in journals provided by the apertures 6 and10.

Each of the gear units has, what is conveniently termed "plus-plus"characteristics, that is to say that they transmit drive by theirmeshing gears rotating in the same direction, rather than in oppositedirections as is the case with conventional or "plus-minus" gears. Wehave now found that plus-plus gearing has the additional characteristicof transmitting drive more efficiently from the larger diameter helicalgear to the smaller diameter helical gear, than from the smaller to thelarger. Thus, turning to FIG. 1, drive is transmitted substantially lessefficiently when in the direction from the smaller gear elements 21 and23 to the larger gear members 28, 30, than when the direction of driveis reversed and drive is transmitted from the gear members 28, 30 to thegear elements 21,13.

The housing 1 is rotated about the common axis of the shafts 26,27 by acrown wheel and pinion (not shown) as is customary with a differentialgear assembly.

OPERATION OF FIRST EMBODIMENT

When the vehicle to which the differential gear assembly is fitted istravelling in a straight line, the gear units 21,28 and 23,30 cannotrotate relative to one another because of their inter-connection throughthe meshing spur gears 16 and 17. The rotational drive of the housing 1is then imparted directly to the output shafts 26 and 27.

If the vehicle is cornering, one of the half shafts, say the half shaft27, will rotate faster than the other half shaft 26. The speed ofrotation of the housing 1 will then be the mean of the speeds ofrotation of the half shafts 26,27. The difference in the rotationalspeeds of the half shafts 26,27 and the housing is accommodated by thegear member 30 driving the gear element 23 in one direction with anacceptable efficiency, while the other gear member 28 drives the gearelement 21 in the opposite direction, also with an acceptableefficiency. This follows from the fact that in both cases the drive istransmitted from the larger diameter gear to the smaller diameter gear.Such counter-rotation of the gear elements 21 and 23 is cancelled out bythe rotation of the two meshing spur gears 16,17 in respectivelyopposite directions. The resultant differentialling action of the gearassembly takes place with the gear units under full driving torque. Theefficiency of transmission of movement from the larger diameter helicalgear members to the smaller diameter gear elements is typically 0.5.

If one of the drive wheels, say the one fitted to half-shaft 27, hasinsufficient ground adhesion to resist the driving torque in the halfshaft, the half-shaft's torque falls accordingly. In a "free"differential in such circumstances there is a corresponding drop intorque in the other half-shaft and any surplus power is expended ingenerating spin in the wheel having poor ground adhesion, against theresistance of the wheel having good ground adhesion. However in thelimited slip differential being described, the frictional forces acrossthe gear assembly act against the equalisation of half-shaft torques insuch circumstances and also act against the generation of wheel spin inthe wheel fitted to half-shaft 27 thereby sustaining an imbalance ofdriving torques between the half-shafts 26 and 27. Transmission oftorque from half-shaft 26 to half-shaft 27 is from gear member 28 togear element 21, against and efficiency of 0.5. The gear element 21transmits the torque through the meshed spur gears 16,17 to the gearelement 23 which meshes with the gear member 30. However this drive isnow transmitted from the smaller diameter gear element to the largerdiameter gear member. As stated above, this is achieved only with a verymuch poorer efficiency which, typically, is only 0.1. The limited slipdifferential described is therefore above to sustain a torque imbalancebetween the two half-shafts in excess of a 90:10 ratio before the onsetof wheel spin.

PREFERRED EMBODIMENT OF THE INVENTION

The embodiment of the invention described with reference to FIGS.3,4,5,7 and 8, operates in the same way as the embodiment justdescribed, but the layout of the assembly is different to enable thetorque applied to each of the half shafts to be shared amongst a groupof four helically-toothed gear elements each of which has more than onetooth convolution in continuous engagement with the associated gearmember.

FIG. 3 shows a differential gear assembly housing 40 in longitudinalsection. A half shaft 41 projects through an end cap 70 on one end ofthe housing 40, and carries a helically-toothed gear member 42 withinthe housing as in the embodiment already described. The opposite end ofthe housing is also fitted with an end cap 71 through which extends ahalf-shaft 48 carrying within the housing 40 a second gear member 47.The two gear members 41,48 are arranged in a spaced coaxial relationshipwith one another and are also coaxial with the axis of rotation of thehousing which is rotatably driven by a conventional crown-and-pinionarrangement (not shown).

The housing 40 has a web 73 spanning diametrically across its centralportion as shown in FIGS . 7 and 8, and is provided with a ring ofaxially-parallel wells 74 on one side and an offset ring ofaxially-parallel wells 76 on the other side. These wells overlap oneanother and, as shown in FIG. 4, accommodate respective pairs of meshingspur gears one spur gear of a pair being referenced 45 and the otherspur gear of the pair being referenced 49. As shown in FIG. 8, a centralaperture 78 in the web 73 provides a bearing for the adjacent ends 79 ofthe shafts 41,48. All of the bearings used are constructed as end-thrustbearings to absorb axial thrusts generated by the rotating components.

Four identical helically-toothed gear elements 43 as shown in FIG. 3,mesh with the gear member 42 at equi-spaced intervals around itscircumference. Each of the elements 43 is of significantly smallerdiameter than the gear member 42. The gear gear elements 43 and the gearmember 42 are of the same hand and rotate in the same sense in thetransmission of drive.

Each of the gear elements 43 carries one of the spur gears 49 at one endas clearly shown in FIG. 5. Each of the spur gears 49 meshes with acorresponding spur gear 45. The spur gears 45 are each mounted at oneend of a gear element 46 which has its axis offset circumferentiallywith respect to the axes of the gear elements 43. Each gear element 46is in mesh with a gear member 47 attached to the half shaft 48projecting from the housing 71 at the opposite end of the housing 40.The extent of offsetting of the gear elements is apparent from FIG. 4.The gear member 42 is identical to the gear member 47. The spur gearpairs 45,49 are separate from one another and each pair is associatedwith a unique pair of gear elements 43,46.

OPERATION OF THE PREFERRED EMBODIMENT

Referring to FIG. 3, the rotational drive applied to the housing 40 isimparted to the half shafts 41,48 to tooth engagement between the gearelements 43,46 and the gear members 42,47.

As long as both driving wheels attached to the half shafts retain roadtraction, the torque output to each of the half shafts will balance atthe spur gears 45,49. These spur gears thus act as fulcrums duringdifferentially. During movement of the vehicle along a straight line,there is no rotation of the gear elements 43,46 relative to the gearmembers 42,47 and the driving torque to the two wheels is equal.

When the vehicle is cornering, the half shaft on the inside of thecorner, say the half shaft 41, will rotate at a slower speed than thatdriving the outer wheel connected to the half shaft 48. The meshinggears in the housing will translate under load to compensate for thedifference in speeds of rotation of the half shafts 41,48 with thehousing 40 rotating at the mean of the two, half shaft speeds. Whilstoutput torque remains in balance at the spur gear fulcrums, there is animbalance in torque between the inner half shaft 41 and the outer halfshaft 48 generated by the friction between the gear elements and thegear members with which they are in mesh.

The meshing gears on both sides of the housing operate at the efficiencyof the gear member driving the smaller diameter gear element. For a gearratio of 2:1, such efficiency would typically be 0.70. The result isthat the torque imparted to the inner half shaft 41 will be the balancetorque at the spur gears supplemented by about 30%, and the torqueimparted to the outer half shaft 48 will be the balance torque at thespur gears minus about 30%.

In traction to one of the drive wheels cannot resist the drive torque inthe half shaft to which it is connected, there will be a tendencytowards wheel spin. If this occurs, it causes frictional forces acrossthe gear assembly to generate an imbalance in the half shaft drivingtorques, whereby the torque in the wheel exhibiting poor traction willbe limited to the traction available at that wheel. The torque to theother wheel is increased to compensate.

The limit to the extent to which such imbalance can be generated isdependent upon the gear efficiency across the assembly as if one wheelwere driving the other. This is referred to as the "torque bias ratio"of the differential which, in the preferred embodiment, is:

1:the efficiency of drive of the gear member driving its associated gearelement, times the efficiency of drive of the gear element when drivingits associated gear member.

The respective efficiencies of drive in opposite directions of drivetransmission between the gear element and the gear member are typically0.70 (as previously stated) and 0.15.

Putting the above values in the torque bias formula given above, thetorque bias ratio becomes:

1: 0.70 times 0.15, or,

1:0.105.

In practical terms, this means that the torque which can be resisted bythe wheel having poor traction, may be multiplied ten times in thetorque to the wheel retaining good traction, always provided that thetraction of this latter wheel is sufficient to resist the increaseddrive torque.

It should also be pointed out that in the above simplified calculationsthe efficiency of the spur wheels has been disregarded as it is close to100%.

It will be noted that each of the gear elements 43,46 in the assemblyshown in FIG. 3 has two convolutions plus overlap in contact with thegear members 42,47. The strength this gives to the drive transmissionfor a given external radius of the housing 40, represents an improvementover prior art systems using crossed helical gearing, as only a singleconvolution plus overlap of the crossing gear can be in mesh at any onetime with its gear member. Also, the number of crossed gears which canbe used in practice is limited by the maximum diameter requirements ofthe housing.

VARIATION OF EITHER EMBODIMENT

FIG. 6 shows a technique which can be employed to reduce end thrusts onthe housing and produced by the plus-plus characteristics of the gearelements in mesh with the gear members. In the figure, two half shaftsare shown at 50,51 which extend respectively from opposite ends of thehousing (not shown). Each half shaft has a gear member 52,53. Each gearmember 52,53 has two axially spaced and helically-toothed sections 54,55and 56,57, respectively. The teeth in the sections 54 and 55 spiral inthe opposite direction to the teeth in the sections 56,57 as isillustrated, but are otherwise the same.

The gear members 52, 53 respectively mesh with gear elements 57,58 whichare of extended length and each has two axially spaced sections in whichthe teeth spiral in opposite senses, as shown, so as to mesh with thecorresponding helically-toothed sections of the gear members 52,53. Theaxially spaced sections of the gear element 57 are referenced 60 and 61,and those of the gear element 58 are referenced 62,63.

As previously mentioned, a characteristic of plus-plus gearing is thatrelatively high end thrusts on their shafts are produced. Thearrangement of toothed sections shown in FIG. 6 divides the load equallybetween each pair of toothed sections and, as the teeth of one sectionextend in the opposite direction to those of the other section of thesame gear, the end thrusts produced by the two sections of each gear actin opposite directions and are therefore self-cancelling.

FIG. 9 shows graphically the way the mechanical efficiency of a gearunit having meshing externally toothed helical gears of differentdiameters, changes with change in diameter of the meshing gears. Theupper family of five curves show the efficiency of transmission of drivefrom the larger diameter gear to a smaller diameter of gear, when theration of the diameters is different. These ratios are referenced R andhave the values 1,2,3,5 and 7 for the respective curves.

A second family of curves is shown in broken outline and gives themechanical efficiencies achieved in the gear units in which R is equalto 1.5,2,3 and 5, when the direction of drive through them is reversed,i.e. drive is transmitted from the smaller gear to the gear of largerdiameter.

An advantage of all of the constructions of differential gear assembliesof the invention and described above, is that all rotational movementoccurs about parallel axes. This is a potentially preferable system tothe TORSEN assembly where components rotating about non-parallel axeshave to be accommodated.

We claim:
 1. A limited slip differential gear assembly, comprising ahousing mounted for rotation about a first axis, means for coupling arotational drive to said housing, two collinear and spaced gear membershaving external teeth and respectively mounted in opposite sides of saidhousing, and at least one pair of gear elements meshing respectivelywith each of the gear members and interconnected by spur gears whichrotate with respective gear elements; in which gear assembly the axes ofrotation of the gear members, the gear elements and spur gears extendparallel to one another and to the axis of rotation of the housing, andthe gear elements and gear members have meshing helical teeth of thesame hand with the gear members being of larger diameter than the gearelements.
 2. An assembly as claimed in claim 1, in which each gearmember has a symmetrical arrangement of gear elements meshing with itand having their axes of rotation lying on the same pitch circle anddisposed between the rotational axes of a similar symmetricalarrangement of gear elements meshing with the second gear member, theassembly also including discrete pairs of meshing spur gears arrangedwith the two spur gears of each pair respectively connected to two gearelements one of which is in mesh with one of the gear members and theother of which is in mesh with the other gear member.
 3. An assembly asclaimed in claim 2, in which the housing has an intermediate webspanning across its interior and formed with cylindrical wellscontaining respective spur gears.
 4. An assembly as claimed in claim 3,in which there are four pairs of spur gears.
 5. An assembly as claimedin claim 1, in which each of the gear elements and the gear members havetwo axially spaced, helically-toothed sections in one of which the teethspiral opposite-handedly to the teeth of the other section.
 6. Anassembly as claimed in claim 1, in which there are at least twoconvolutions of meshing teeth between each gear element and each gearmember.
 7. A limited slip differential gear assembly comprising:ahousing mounted for rotation about a first axis, the housing having anintermediate web spanning across its interior and formed withcylindrical wells, means for coupling a rotational drive to saidhousing, first and second collinear and spaced gear members havingexternal helical teeth and respectively mounted to opposite sides ofsaid intermediate web for rotation about axes that are parallel to saidfirst axis, a first group of gear elements having external helical teethof the same hand as the first gear member and meshing with the firstgear member, the gear elements of the first group being of smallerdiameter than the first gear member, being symmetrically arrangedrelative to the first gear member, and having their axes of rotationparallel to said first axis and lying on a pitch circle, a first groupof spur gears which are connected to and rotate with respective gearelements of the first group about axes that are parallel to said firstaxis and are contained in respective wells of the intermediate web, asecond group of gear elements having external helical teeth of the samehand as the second gear member and meshing with the second gear member,the gear elements of the second group being of smaller diameter than thesecond gear member, being symmetrically arranged relative to the secondgear member, and having their axes of rotation parallel to said firstaxis and lying on said pitch circle and disposed between the axes ofrotation of the gear elements of the first group, a second group of spurgears which are connected to and rotate with respective gear elements ofthe second group about axes that are parallel to said first axis and arecontained in respective wells of the intermediate web, each spur gear ofthe second group being paired with a single spur gear of the first groupand being meshed therewith, whereby each gear element of the first groupis connected to a gear element of the second group.
 8. An assembly asclaimed in claim 7, wherein the intermediate web is formed with a firstgroup of cylindrical wells open toward the first gear member andcontaining the first group of spur gears respectively and a second groupof cylindrical wells open toward the second gear member and containingthe second group of spur gears respectively.
 9. An assembly as claimedin claim 7, wherein each spur gear and the gear element to which it isconnected are formed by a single body that rotates about an axis that isparallel to said first axis.
 10. An assembly as claimed in claim 7, inwhich each group of spur gears comprises four spur gears.
 11. Anassembly as claimed in claim 7, in which each gear element has twoaxially-spaced, helically-toothed sections of opposite hand, and thegear member meshing therewith has two axially spaced, helically toothedsections of opposite hand meshing with the two sections respectively ofthe gear element.
 12. An assembly as claimed in claim 7, in which thereare at least two convolutions of meshing teeth between each gear elementand the gear member with which it meshes.
 13. A limited slipdifferential gear assembly comprising:a housing mounted for rotationabout a first axis, means for coupling a rotational drive to saidhousing, first and second collinear and spaced gear members havingexternal helical teeth and mounted in said housing for rotation aboutaxes that are parallel to said first axis, at least one pair of firstand second gear elements each having an axis of rotation parallel tosaid first axis, the first gear element being of smaller diameter thanthe first gear member and having an external helical tooth of the samehand as the first gear member and meshing with the first gear member,and the second gear element being of smaller diameter than the secondgear member and having an external helical tooth of the same hand as thesecond gear member and meshing with the second gear member, a first spurgear which is connected to and rotates with said first gear elementabout an axis that is parallel to said first axis, a second spur gearwhich is connected to and rotates with said second gear element about anaxis that is parallel to said first axis, said second spur gear beingmeshed with said first spur gear, whereby the first gear element isconnected to the second gear element.
 14. An assembly as claimed inclaim 13, comprising a plurality of pairs of first and second gearelements, the first gear elements of the pairs respectively having asymmetrical arrangement around the first gear member and having theiraxes of rotation on a pitch circle, and the second gear elements of thepairs respectively having a symmetrical arrangement around the secondgear member and having their axes of rotation between the axes ofrotation of the first gear elements and lying on said pitch circle. 15.An assembly as claimed in claim 14, comprising a plurality of discretepairs of matching spur gears with the two spur gears of each pairrespectively connected to a first gear element and a second gearelement.
 16. An assembly as claimed in claim 15, in which the housinghas an intermediate web between the first gear member and the secondgear member.
 17. An assembly as claimed in claim 16, wherein theintermediate web is formed with cylindrical wells containing the spurgears respectively.
 18. An assembly as claimed in claim 16, in whichthere are four pairs of spur gears.
 19. An assembly as claimed in claim13, in which each gear element has two axially-spaced, helically-toothedsections of opposite hand, and the gear member meshing therewith has twoaxially spaced, helically toothed sections of opposite hand meshing withthe two sections respectively of the gear element.
 20. An assembly asclaimed in claim 13, in which there are at least two convolutions ofmeshing teeth between each gear element and the gear member with whichit meshes.